This invention discloses a new technique for gusseting continuous tubing wherein the material of the tubing is of a generally imperforate character to the free passage of gas or air.
The invention particularly applies to plastic film or sheet tubing since this material is frequently gusseted along one or more edges, in the common practice of converting the tubing into plastic bags of a type incorporating a gusset structure. The term "gusseting" as used in this disclosure refers to art of tucking an edge(s) of the tubing inwardly along a continuous longitudinal section thereof such that the cross section of the edge resembles, for example, a flattened "W", or alternately a series of W's where several adjacent and distinct inward tucks are performed.
The past standard practice for gusseting plastic tubing is exemplified by the steps of injecting inflating gas into the tube, to expand the same to a generally full condition, pinching the inflated tube such as by nip rolls to trap the gas pressure therewithin, and thereby establish an inflated section of tubing just prior to the indicated nip rolls, and while advancing the tube, applying a gusseting blade(s) or the like to a side portion of such inflated section to continuously tuck such side portion inwardly.
Frequently such gusseting steps are performed simultaneously with the extrusion of the tubing by the so-called "bubble" process. As a characterizing step of this process, the plastic melt is extruded as continuous inflated tube, and at a point remote from the extrusion die, the tube is collapsed to trap the inflating gas therewithin, thereby forming a bubble or inflated section in a sense suitable for the noted gusseting procedure.
As is inherent to this procedure, the gusseting blade(s) places a physical drag on the somewhat delicate tube, which, of course, varies in degree depending largely on the speed of manufacture, the relative slip values along the contacting surfaces, the amount of contact, and the extent of firmness or resistance of the tube to the inward deformation forced thereon by the blade(s). Understandably, a certain amount of drag can be tolerated. However, particularly as the speed of manufacture is increased to maximum attainable rates, such as is possible using liquid quenching or cooling practices, the drag produced by the blades can build to a level that it becomes a destructive and/or unstabilizing force in the manufacture of the film.
Moreover, the apparent efficiencies of utilizing the bubble formed off the extrusion die for gusseting, can be offset in instances where the film is to be cooled by liquid quenching, particularly in the manner shown in U.S. Pat. Nos. 3,090,998 and 3,337,663. Here the folds of the gusset tend to capture and entrain droplets of the quenching liquid, which can subsequently interfere with heat sealing the film unless the film is first properly dried. Since the droplets are concealed within the folds of the gusset, the film is not as easily dried in the area of the gussets, such as by forced air drying. As a result the film may not be properly ready for immediate conversion into plastic articles, such as plastic bags.
Accordingly, it would be of advantage if there were made available to the art, an improved technique for forming continuous longitudinal gussets in tubular material, and particularly a technique which placed minimal physical drag on the material in effecting the gusseting steps.
It would be of even further advantage in such art, particularly in certain instances of manufacture, if drag against the side of the tube could be essentially eliminated, by eliminating the customary gusseting blade(s), as has been regularly employed in the past to form the inward tuck required to gusset the tube.
It would be of yet further advantage to the art, if tubular film material could be effectively gusseted utilizing a minimal internal pressure to prepare the film for gusseting.
It would be of still further advantage to such art, if such gusseting steps could be readily performed in line with the manufacture of the material, and in a manner noninterfering such as with liquid quenching techniques, where practiced.
It would be of yet even further advantage if in instances where desired or most practical, such gusseting steps could be performed exclusive of the mannufacturing operation, as for example, for purposes of gusseting continuous lengths of tubular material fed from a compact storage source, such as from a mill or storage roll.